Protein kinases represent a large family of proteins, which play a pivotal role in the regulation of a wide variety of cellular processes, maintaining control over cellular function. Protein tyrosine kinases may be classified as growth factor receptor (e.g. VEGFR, EGFR, PDGFR, FGFR and erbB2) or non-receptor (e.g. c-src and bcr-abl) kinases. The receptor type tyrosine kinases make up about 20 different subfamilies. The non-receptor type tyrosine kinases make up numerous subfamilies. Receptor tyrosine kinases are large enzymes that span the cell membrane and possess an extracellular binding domain for growth factors, a transmembrane domain, and an intracellular portion that functions as a kinase to phosphorylate a specific tyrosine residue in proteins and hence to influence cell proliferation. Aberrant or inappropriate protein kinase activity can contribute to the rise of disease states associated with such aberrant kinase activity.
A partial list of such kinases include abl, AATK, ALK, Aid, axl, bmx, bcr-abl, Blk, Brk, Btk, csk, c-kit, c-Met, c-src, c-fins, CDK1, CDK2, CDK3, CDK4, CDKS, CDK6, CDK7, CDK8, CDK9, CDK10, CRaf1, CSF1R, CSK, DDR1, DDR2, EPHA, EPHB, EGFR, ErbB2, ErbB3, ErbB4, Erk, Fak, fes, FER, FGFR1, FGFR2, FGFR3, FGFR4, FGFRS, Fgr, flt-1, Fps, Frk, Fyn, GSG2, GSK, Hck, ILK, INSRR, IRAK4, ITK, IGF-1R, INS-R, Jak, KSR1, KDR, LMTK2, LMTK3, LTK, Lck, Lyn, MATK, MERTK, MLTK, MST1R, MUSK, NPR1, NTRK, MEK, PLK4, PTK, p38, PDGFR, PIK, PKC, PYK2, RET, ROR1, ROR2, RYK, ros, Ron, SGK493, SRC, SRMS, STYK1, SYK, TEC, TEK, TEX14, TNK1, TNK2, TNNI3K, TXK, TYK2, TYRO3, tie, tie2, TRK, Yes, and Zap70. Inhibition of such kinases has become an important therapeutic target. Certain diseases are known to be associated with deregulated angiogenesis, for example, ocular neovascularisation, such as retinopathies (including diabetic retinopathy); age-related macular degeneration; psoriasis; hemangioblastoma; hemangioma; arteriosclerosis; inflammatory diseases, such as a rheumatoid or rheumatic inflammatory disease, especially arthritis (including rheumatoid arthritis); or other chronic inflammatory disorders, such as chronic asthma; arterial or post-transplantational atherosclerosis; endometriosis; and neoplastic diseases, for example so-called solid tumors and liquid tumors (such as leukemias).
Angiogenesis is an important component of certain normal physiological processes such as embryogenesis and wound healing, but aberrant angiogenesis contributes to some pathological disorders and in particular to tumor growth. VEGF-A (vascular endothelial growth factor A) is a key factor promoting neovascularization (angiogenesis) of tumors. VEGF induces endothelial cell proliferation and migration by signaling through two high affinity receptors, the fms-like tyrosine kinase receptor, flt-1, and the kinase insert domain-containing receptor, KDR. These signaling responses are critically dependent upon receptor dimerization and activation of intrinsic receptor tyrosine kinase (RTK) activity. The binding of VEGF as a disulfide-linked homodimer stimulates receptor dimerization and activation of the RTK domain. The kinase activity autophosphorylates cytoplasmic receptor tyrosine residues, which then serve as binding sites for molecules involved in the propagation of a signaling cascade.
Disruption of VEGF receptor signaling is a highly attractive therapeutic target in cancer, as angiogenesis is a prerequisite for all solid tumor growth, and that the mature endothelium remains relatively quiescent (with the exception of the female reproductive system and wound healing). A number of experiment approaches to inhibiting VEGF signaling have been examined, including use of neutralizing antibodies receptor antagonists, small molecule antagonists, antisense constructs and dominant-negative strategies (“Molecular basis for sunitinib efficacy and future clinical development.” Nature Review Drug Discovery, 2007, 6, 734; Angiogenesis: “an organizing principle for drug discovery?” Nature Review Drug Discovery, 2007, 6, 273).
Hepatocyte growth factor (HGF), also known as scatter factor, is a multifunctional growth factor that enhances transformation and tumor development by inducing mitogenesis and cell motility. In order to produce cellular effects, HGF must bind to its receptor, c-Met, a receptor tyrosine kinase. c-Met is overexpressed in a significant percentage of various types of human cancers and is often amplified during the transition between primary tumors and metastasis. c-Met is also implicated in atherosclerosis and lung fibrosis (“Molecular cancer therapy: can our expectation be MET.” Euro. J. Cancer, 2008, 44, 641-651). Invasive growth of certain cancer cells is drastically enhanced by tumor-stromal interactions involving the HGF/c-Met (HGF receptor) pathway. Binding of HGF to c-Met leads to receptor phosphorylation and activation of Ras/mitogen-activated protein kinase (MAPK) signaling pathway, thereby enhancing malignant behaviors of cancer cells. Moreover, stimulation of the HGF/c-Met pathway itself can lead to the induction of VEGF expression, itself contributing directly to angiogenic activity (“From Tpr-Met to Met, tumorigenesis and tubes.” Oncogene. 2007, 26, 1276; “Targeting the c-Met Signaling Pathway in Cancer.” Clin. Cancer Res. 2006, 12, 3657; “Drug development of MET inhibitors: targeting oncogene addiction and expedience.” Nature Review Drug Discovery, 2008, 7, 504).
Insulin-like growth factor 1 receptor (IGF1R) is an integral membrane tyrosine kinase receptor that binds insulin-like growth factor (IGF) with high affinity. IGF1R plays a critical role in transformation events and human cancer. It is highly over-expressed in most malignant tissues where it functions as an anti-apototic agent by enhancing cell survival through the PI3K pathway, and also the p53 pathway. IGF1R has been linked to various disease states, such as breast and ovarian cancer, metastatic uveal melanoma, macular degeneration, and intrauterine growth retardation and poor postnatal growth, among others (“IGF1R signaling and its inhibition.” Endocrine-Related Cancer, 2006, 13, S33-S43; “The new kid on the block(ade) of the IGF-1 receptor.” Cancer Cell, 2004, 5, 201.).
Anti-tumor approaches that target VEGF/VEGFR, HGF/c-Met and/or IGF/IGF1R signaling may circumvent the ability of tumor cells to overcome VEGFR, HGFR or IGF1R inhibition alone and may represent improved cancer therapeutics. Here we describe small molecules that are potent inhibitors of protein tyrosine kinase activity, such as that of, for example, the VEGF receptor KDR, the HGF receptor c-Met, and/or the IGF receptor IGF1R, among others.